PHYIUM ANNELIDA 273 



around a single stout aciculum to which numerous small muscles are 

 attached. The chaetae and acicula are made of chitin, which resembles 

 the material that forms the exoskeletons of arthropods. 



For walking each parapodium is extended forward and downward, 

 moved backward, withdrawn, then moved upward and forward again. 

 In walking, the movements of the parapodia of each segment are slightly 

 ahead of those on the next anterior segment, producing the appearance 

 of waves of motion that pass forward along the sides. 



In Lumbricus every body segment except the peristomium bears 

 four pairs of chaetae (Fig. 15.5), each of which has small muscles that 

 can move it out or in, and slant it forward or backward. The location 

 of pairs corresponds with the location of the notopodia and neuropodia 

 of the polychaete. These chaetae are used for gripping the sides of the 

 burrow, to assist locomotion. They can be slanted forward or backward 

 to help the worm resist being pulled from the burrow. 



The anus is located on the terminal segment, which always remains 

 the terminal segment as new segments are formed from its anterior edge. 

 In Nereis the parapodia of this segment are reduced to a pair of ventral 

 cirri (Fig. 15.1) which are longer than those of other segments and func- 

 tion as a pair of posterior tentacles. 



120. Nereis and Lumbricus: Body Wall 



The body wall is made of the same layers in both species (Fig. 15.5). 

 The epidermis of Lumbricus has more sensory cells than that of Nereis, 

 a reflection, perhaps, of the lack of sense organs. The musculature is 

 better developed in Lumbricus. In Nereis the circular layer thins out 

 dorsally and ventrally, while in Lumbricus it remains relatively thick. 

 The longitudinal muscles in Nereis are restricted to four bands, whereas 

 in Lumbricus they form a nearly continuous layer. The two musculatures 

 are, however, very similar in general plan. 



The muscles are used differently in the two species. Nereis walks 

 with its parapodia, but often assists them with lateral undulations 

 of the body that pass as waves forward along the body. Nereis can 

 also swim, and then these undulations simply become more vigorous. In 

 its burrow Nereis circulates water by vertical undulations of the body, 

 the waves passing backward along the body to draw water in from the 

 front. All of these sinuous movements involve the longitudinal muscles, 

 which act alternately within a given segment, contracting first on one 

 side and then on the other. The circular muscles are used to increase 

 the length of the body, and are used with the other muscles in digging. 



Lumbricus crawls forward by extending the body, gripping the 

 surface with its chaetae, and then shortening the body. As it moves, 

 coordinated waves of extension and contraction pass posteriorly along 

 the body. The pattern can be reversed so that the waves pass forward, 

 in which case Lumbricus crawls backward. Movement in the burrow is 

 similar but more efficient, since the entire circumference of the worm 

 can be used for gripping. In all of these movements the muscles of a 



